1.1. Study Background
The increasing consumption of natural resources and primary energy and its accompanying greenhouse gas (GHG) emissions is considered a threat to environmental sustainability [
1]. It is widely recognized that the building sector is a major culprit of materials and energy consumption and therefore possess huge potentials to reduce global emissions through energy efficiency improvement and renewable energy applications [
2,
3,
4]. Particularly, the building sector consumes about 30% of primary energy globally and 40% in Europe and the United States with an equivalent share of GHG emitted [
5,
6]. In 2014, the Paris Agreement sought a global effort to reduce global warming to 1.5 degrees Celsius, compared to pre-industrial levels [
7]. Participating countries were required to submit nationally determined contributions and long-term low greenhouse gas emission development strategies in 2020. For instance, the European Union has set goals to increase energy efficiency to at least 32.5% and the share of renewable energy by at least 32% by 2030 [
8]. In Asia, Hong Kong has also set an ambitious carbon reduction of 26 to 36% by 2030 using 2005 as the baseline [
9]. These targets have expedited the exploitation of energy-efficient building design and technologies in such developed countries. However, it is not prevalent in developing countries such as those in sub-Saharan Africa.
In order to accelerate global carbon emission reduction, the UN has emphasized the need for ambitious plans to drastically improve energy efficiency and increase the share of renewable energy sources in the latter region [
10]. Various targets encompassing final energy generation mix, heating and cooling requirement, and transportation are being introduced at various levels of governance. Comparatively, renewable power has received the vast majority of attention whereas targets for energy-efficient cooling and heating of buildings has been introduced to a lesser degree [
10]. It is well known that drastic improvement in building designs is a critical pathway to achieving these targets. Although practical, it is presently less feasible due to the lack of localized energy efficiency building codes and standards in the sub-Saharan Africa region [
11]. In Ghana, the residential sector consumed about 46% of electricity generated in 2019 [
12]. This demand is projected to increase by 5.8% annually due to the increasing population coupled with poorly designed buildings [
12]. Presently, frequent power outages underscore the inefficiencies in supply-side management. Although demand-side management possesses huge potentials to resolve this energy deficit, the Energy Commission of Ghana has focused on increasing the generation capacity. However, this has been evidently futile as a result of persistent load shedding and power outages in the region. Moreover, the current supply-side management is expensive and unsustainable due to the increasing use of thermal sources (62.7%) [
12]. These prevailing conditions therefore necessitate studies which investigate strategies to reduce building energy consumption in the sub-Saharan Africa region.
1.2. Literature Review
Successful efforts to reduce building energy demand in developed countries have been mainly driven by passive and active design strategies. Typical passive design strategies explored include the shape of the building, the orientation of the building, construction materials, window-to-wall ratio, and ceiling height whereas active strategies include efficient energy systems and renewable energy generation. The integration of parametric design through building information modelling (BIM), building energy simulation and optimization methods enable the transformation of building design problems into an optimization problem in order to explore the aforementioned strategies [
13]. For instance, Al-Saadi and Al-Jabri [
14] employed a computational approach to optimize the envelope design of buildings in hot climates. The study evaluated envelope design strategies including envelope airtightness, roof, and wall insulation, thermal mass, window area, window glazing and window shading. The results showed that the window shading is thermally and economically beneficial in the studied hot climates while insulation of 2–5 cm is cost-optimal. Saroglou et al. [
15] explored the impact of three single-skin façades and ventilated double skin façade on the energy performance of low carbon high-rise buildings. The study further explored the interaction between façade types, orientation, and height which showed varying energy demand reduction at different floors within the building. Additionally, the ventilated double skin façade design with low glazing can reduce cooling loads by 15%. Another study proposed a building design optimization process to evaluate daylighting and energy performance of multiple design options in order to generate an optimized design [
16]. Design parameters included building depth, roof ridge location, skylight width, height, and orientation, windows, and shading. An office building was optimized which revealed the trade-off between daylighting and energy performance. Ilbeigi et al. [
17] developed a method to optimize building energy consumption using artificial neural network and a genetic algorithm. The proposed method was applied to a case building in Iran by exploring the thermophysical properties of walls, floor, roofs, and windows which reduced energy demand by 35%.
A study developed an integrated decision-making strategy to optimize energy demand, energy production, and adaptive thermal comfort towards a zero-energy high-rise building [
18]. Their approach involved a two-stage optimization of design parameters including window-to-wall ratio (WWR), wall u-value, glazing construction, airtightness of the façade, cooling setpoint and photovoltaic (PV) surface area which reduced building energy demand by 33%. Cuce et al. [
19] evaluated natural ventilation as a passive cooling strategy for educational buildings by exploring design strategies such as the depth of the room, atrium, solar chimney and orientation. The results indicated that proper design of single-sided and cross-ventilation can ensure cooling and improve the air quality in the case study building. Acar et al. [
20] focused on optimizing building envelope parameters to reduce the energy use and cost of residential buildings in Turkey. The design variables investigated include wall material types and thickness, external wall, roof, and floor insulation, glazing types, and window dimensions. The results indicated that the most energy-efficient solutions also led to the lowest lifecycle cost. Cuce and Riffat [
21] also validated the thermal performance of a commercially available vacuum glazing using CFD-based simulation. The study also explored the application of translucent aerogel support pillars in vacuum glazing which showed an improvement of the U-value by up to 67%. Cuce et al. investigated the thermal, energy, and optical performance of heat insulation glass under different cooling and heating season through laboratory and in situ tests [
22]. The results indicated that in comparison to conventional single glazing, the heat insulation glass provides energy savings of about 48% and 38% during cooling and heating seasons respectively. Furthermore, a 16% higher generation can be achieved due to its nanolayer reflective film. In another study [
23], the thermal performance of a novel vacuum tube window was investigated through numerical and experimental investigations which showed a higher thermal resistance in comparison to other commercialized windows.
With regards to integrated photovoltaic applications, Chen et al. [
24] optimized the thermal and optical properties of photovoltaic façades and conventional passive parameters for a high-rise building in Hong Kong. The results showed that the window geometry, thermal, and optical properties were the most important parameters and a net energy demand reduction of about 49% was achieved. Chen et al. [
25] also developed a holistic design optimization process to investigate the interactions between photovoltaic façades and traditional passive design parameters. A case study in Hong Kong revealed that a net energy demand reduction of about 71% could be achieved with the optimum design configuration. Another study optimized the performance of a dynamic photovoltaic system for adaptive shading which showed a net energy saving of up to 80% [
26]. Skandalos and Karamanis [
27] also explored the optimal configuration of different building-integrated photovoltaics under different climatic zones. The optimal building integrated photovoltaic (BIPV) configuration achieved energy savings of up to 43%.
It is clear from the foregoing that huge energy savings can be achieved during the operational stages through passive and active strategies. However, the increasing use of low energy materials especially within the building envelope may have a counteracting effect on energy savings since such materials have high embodied energy [
28]. Some studies suggest a point after which further operational energy saving leads to exponential increases in embodied energy [
29]. In fact, concerning low-energy buildings, over 45% of the lifecycle energy could be due to the embodied energy use, thus from a lifecycle perspective, the embodied energy could override the operational energy savings [
30]. It is therefore crucial to minimize operational energy use without compromising the embodied energy performance. Thus, a properly optimized building envelope must ensure an improved energy performance from a whole lifecycle perspective. The development of low-energy buildings from a lifecycle perspective is becoming an intense research topic [
31]. Tushar et al. [
32] developed an evidence-based integrated framework to optimize the energy and environmental performance of a residential building from a lifecycle perspective. The study first developed scenarios to optimize passive design parameters including building orientation, shading, infiltration rate and window glazing. Following this, the optimized building designs were developed in Autodesk Revit BIM tool and connected with Tally (a BIM-enabled lifecycle assessment (LCA) tool) to evaluate the environmental impacts of the alternative designs. The results of a case study in Melbourne showed that only a moderate level of insulation was required to achieve optimal energy efficiency and also revealed the best insulation materials for minimal environmental impacts. In these studies, the embodied energy is estimated separately and its confounding effects on the operational energy is not explored.
A few studies have focused on joint optimization of the operational and embodied energy of building from a lifecycle perspective. For instance, Lin et al. [
33] developed a multi-objective optimal building envelope model to design the optimal configuration of building envelopes and air conditioning systems in Taiwan. The model explored design variables such as windows, sunshade, glazing, wall and roof materials, and air conditioning systems to reduce the construction cost and the environmental impact of material manufacture. The optimized case reduced carbon emissions by 58.3% while increasing cost by only 5.3%. Kiss and Szalay [
34] developed a modular parametric lifecycle optimization framework for a multi-apartment house in Hungry. The study evaluated design strategies including building geometry, envelope, fixtures, and heating energy source which produced up to an 80% reduction in environmental impacts. The result also showed that optimizing the operational or embodied impacts alone leads to a sub-optimal design. Shadram and Mukkavaara [
35] integrated BIM with a multi-objective optimization approach to explore the trade-off between embodied and operational energy uses of new building projects. The method was tested on a low energy dwelling in Sweden which showed that small operational energy savings could lead to larger increases in embodied energy. Shadram et al. [
29] also developed a multi-objective optimization approach to explore a wide range of retrofitting solutions for a Swedish multi-family residence from a lifecycle energy perspective. The study investigated retrofitting measures including material types, materials quantities, window types, and HVAC systems to optimize building design solutions that satisfy the Swedish energy-efficient building codes by exploring a trade-off between embodied and operational energy uses. Abbasi and Noorzai [
36] combined a multi-objective optimization method with BIM and LCA to determine the optimal trade-off between embodied energy and operational energy of a residential building in Iran. The results showed that up to 65% of energy savings can be achieved through renewable energy.
1.3. Objectives and Novelty of Study
The literature review shows that there is a limited number of studies that jointly optimize the embodied impacts of buildings from a lifecycle perspective. Most studies focus on design parameters for the operational stage or perform a stepped assessment in which the operational stage is solely optimized, followed by a simplified estimation of the embodied impacts of the building. In doing so, the impacts of confounding parameters in a joint optimization remain unexplored. The integration of building information modelling and optimization methods provides promising prospects to extensively explore the trade-off between operational and embodied impacts. Moreover, there are no identified studies to optimize indigenous façade types and buildings archetypes in sub-Saharan Africa, where poor envelope designs lead to large heat gains through the building envelope. Due to the extremely hot weather conditions in these areas, sustainably designed envelopes are critical to reducing overall lifecycle energy consumption. It is therefore necessary to understand how envelope design strategies affect the overall lifecycle energy performance of the building.
The aim of this study is to determine the optimal configuration of different building envelopes with local materials that are representative of the climatic conditions sub-Saharan region of Africa from a whole lifecycle perspective. A genetic algorithm is coupled with building energy simulation and LCA to explore the optimal performance of different building envelopes and design variables under different climatic conditions in the sub-Saharan Africa Region. Major contributions of the study include an integrated optimization that reflects the whole lifecycle performance of buildings. The optimization model is properly defined to include and solve the counteracting effects of building materials on the trade-off between operational and embodied energy in the lifecycle of buildings. Additionally, a comprehensive exploitation of the performance of local building envelope materials under the diverse climate of sub-Saharan Africa can facilitate the adoption of energy-efficient building envelopes. Buildings contribute over one-third of the energy use in these regions, therefore improvement in energy efficiency is crucial to reduce the overall energy consumption. Furthermore, the introduction of building-integrated photovoltaics will increase the share of renewables in the total electricity mix.
The remaining content of this paper is divided into three sections.
Section 2 presents the modelling of building energy use based on different design variables and façade configurations before the optimization method.
Section 3 shows the performance of each building façade and the interaction between design variables through a case study. Finally, conclusions and recommendations are given for the sub-Saharan African region in
Section 4.